The proposed studies have been designed to utilize the micropuncture technique in a systematic evaluation of uric acid transport along the length of the proximal tubule of the rat kidney. A radioassay for uric acid-2-14C has been developed in our laboratory for this purpose. Utilizing this method, free flow micropuncture experiments have demonstrated bidirectional transport of uric acid in the proximal tubule with net reabsorption of uric acid apparent early and net secretion evident later in this nephron site. Virtually no net transport of uric acid could be detected beyond the accessible portion of the late proximal tubule. As an extension of these findings, the long-term goal of this project is to assess further the opposing proximal tubular transport processes by investigating those factors which might influence urate reabsorption and/or secretion. Studies have been designed to evaluate the separate transport processes under conditions of acute and chronic elevations in plasma urate concentration, acute and chronic hemodynamic alterations, variations in acid-base balance and the administration of urate retaining and uricosuric drugs. It is anticipated that these studies may prov!de some insight into the nature and mechanism of the separate transport processes and their possible interrelationship.